1. Field of the Invention
The present invention relates to an image transmission system and the method by or in which an image scene is divided into a plurality of blocks, and video signals are transformed for each block in accordance with an orthogonal transform method and then coded before transmission.
2. Description of the Prior Art
As one of digital image transmission systems available for visual telephone or conference applicances, an orthogonal transform coding system is well known. In this system, an image frame is divided into a plurality of blocks, and a series of numerical sampled values transduced at picture elements are orthogonal transformed for each block, coded and then transmitted. On the receiver side thereof, the received digital signals are inversely transformed to reproduce video signals for each block, and then a complete frame is reproduced by integrating or scanning the divided video signals. The above system is described in detail in "DIGITAL IMAGE PROCESSING" published from NIKKAN KOGYO CO., by Masahiko Kiyuki, therefore which is incorporated herein by reference.
In this transformation method, cosine transform, Hadamard transform, KL transform, Harr transform, etc. are all adoptable. However, it has been considered that the most efficient coding system can be attained in accordance with Cosine transformation method. An example of the systems including a cosine transformer and an adaptive coder is described in "Adaptive Coding of Monochrome and Color Images" by When-Hsiuns Chen and Harrison Smith, IEEE TRANSACTION 0N COMMUNICATIONS, VOL. COM-25, No. 11, NOV. 1977. The principle of this orthogonal transform coding system will briefly be described hereinbelow. First, video signals are transduced at picture elements and A/D converted, and then the converted digital video signals are divided into a plurality of blocks each composed of 64 picture elements of vertical 8 lines and horizontal 8 pixels, for instance. The divided video signals are cosine transformed block by block, and then the cosine transform coefficients are coded and transmitted. On the receiver side, the coded cosine transform coefficients are decoded, inversely cosine transformed to obtain video signals for each block, and then the video signals of each block are integrated by use of a memory unit. Thereafter, the integrated video signals are scanned to reproduce digital video information signals for each frame, and then D/A converted to obtain the original analog video signals.
By the way, it is well known that power (the product of the amplitude and the time) of the transformed cosine coefficients is high in specific components (low-order cosine coefficients) on the basis of the statistical characteristics of video signals. Therefore, in this method, video signals are quantized by allocating many bits to higher power components and small bits to lower power components under consideration of human visual characteristics in order that the transmission speed can be reduced and the coding efficiency can be improved. Recently, Codec has reported an orthogonal transform coding system with a transmission speed of 1.5 Mbps.
This Codec orthogonal transform coding system has fairly improved the coding efficiency; however, the transmission speed required for image transmission is still high as compared that required for audio or other data signals, thus resulting in a problem in that the image communications are still high in cost.
In this connection, the data transmission speed is 64 kbit/sec in audio signal but as high as 100 Mbit/sec in video signal. Therefore, ordinary CMOS logic circuits can be adopted for audio signal transmission; however, ECL (emitter coupled logic) circuits should be adopted for video signal transmission.